This invention relates to circuit breaker closing systems, and more specifically relates to a novel synchronous closing system for extra-high voltage circuit breakers.
It is known that extra-high voltage circuit breakers, for example, those operating in lines of 550 kV or more should preferably be closed in synchronism with the closing resistor current zero. This will limit switching surge overvoltages on the line due to trapped charges on the line and due to the main contact shunting by a closing resistor.
To obtain synchronism between the contact closing and the current zero, it is known to provide zero cross-detection systems which monitor the resistor current and which produce closing signals which are applied to hydraulic operating mechanisms for the circuit breaker with a timing such that the hydraulic operating mechanism will close the circuit breaker contacts at an instant in a two millisecond "window" around a zero current.
It has been found that such systems do not give good repeatability. Variable closing instants have been found to be caused by changes in ambient temperature, inconsistency in the operation of the control valve, inconsistency in the movement of the piston, and variations in the control voltage controlling the various elecrtrical relays and other components in the system. In particular, it was found that the rate of rise of pressure for operating the pneumatic piston varied considerably and this caused variable and unpredictable changes in the closing instant which were, in some cases, unacceptably greater than the desired two millisecond window for closing around the zero current instant.